Tuesday, July 31, 2007

Slay the Beast

Per CPC data, sea surface temperature anomaly (SSTA) magnitudes throughout much of the global tropics are still generally less than 2C with warmth across the Indian and west central Pacific Oceans and coolness along the east Pacific equatorial cold tongue. Exceptions include ~2-3C just east of Africa and west of South America. Totals across the central Indian and west central Pacific Oceans are ~29-30C. Recent 5-day averaged TAO buoy data suggests considerable warming of subsurface anomalies along the equatorial east Pacific cold tongue, with magnitudes ~minus 1C at roughly 100m. The horseshoe spatial distribution of warm SSTAs across the west Pacific Ocean also persists. The largest anomalies and warmest SSTs are across the tropical northwest Pacific (TNWP) from the region of the South China Sea (SCS) to west of the date line. Totals remain ~30-31C.

As discussed below, the global circulation continues to behave in a La-Nina like manner (which may be traced back to subseasonal events boreal fall 2006). However, as I have been stating for a few weeks, my own feelings are the coolest SST anomalies will remain across the East Pacific and this will not be a basin wide situation. The SSTs across the west central and TWNP as well as much of the Indian Ocean are warmer than normal and will continue to impact the global circulation. The positive SST anomalies across the west Pacific warm pool are deep and have been persistent for at least the past 5-7 years.

In contrast to some past writings, I only want to summarize recent subseasonal forcing-response-feedbacks, etc. for brevity. Several monitoring tools including renditions of Wheeler and Hendon (2004) phase space plots from various international operational weather centers (ESRL/PSD MJO web page) indicate the MJO signal has virtually collapsed. We need to be careful what we are trying to interpret.

A convectively coupled Kelvin wave moved into the west central Pacific Ocean over the last week or so. Extratropical Rossby wave energy dispersions from East Asia interacting with the convection produced a local environment favorable for the development of Typhoon Usagi. I think this Kelvin wave signal is moving across the equatorial Western Hemisphere and may be contributing the recent increase of the Andes Mountain torque to ~plus 20 Hadleys per 28 July R1 data. However, I also think the global circulation component of what may still be a MJO signal remains across the Eastern Hemisphere centered ~90-100E. Various animations of wind fields support this notion, with loosely twin anomalous upper tropospheric tropical/subtropical anticyclones from Africa into the Indian Ocean and twin cyclones across eastern Indonesia to the date line. I think the tropical convection will respond to these circulation anomalies meaning a reorganization and slow east-northeast shift (coherent modes Hovmollers support this).

Full disk satellite imagery presents a nice signal this process is occurring. Intense tropical rainfall currently extends from India east-southeast to the equatorial west central Pacific Ocean, having 3-day averaged OLRA ~minus 50-90 W/m**2 centered near 5-10N/110E. I think it is probable to have very intense tropical convection across the warm SSTs of the northwest Pacific Ocean by roughly week 3. This may involve a consolidation of tropical forcing from both the west central equatorial Pacific and a slowly eastward propagating mode.

Total global AAM (28 July) is ~3 standard deviations below the R1 data climatology. Only around 1 March 2007 may have been lower than this value during the past 12 months. The low AAM is not only for the troposphere but also includes the easterly phase of the QBO and the earth component. Irrelevant of how anyone wants to do a real-time earth-atmosphere AAM budget, deep anomalous zonal mean easterly flow dominates much of the tropical and subtropical atmospheres, with incursions into the midlatitudes during July. From a zonal mean perspective, existing higher latitude westerly wind flow anomalies poleward of the easterlies have supported strong midlatitude ridges, including the wicked ridge of the western USA this summer. This circulation situation is consistent with La-Nina (GSDM Stage 1).

Currently both global frictional and mountain torques are increasing. These dynamical processes are already starting to add westerly flow to the atmosphere. As the tropical convection shifts into the northwest Pacific, I think this behavior through interactions with the extratropics and the earth’s surface will add additional westerly flow to the subtropics. This should result in a strong positive AAM tendency and the Global Wind Oscillation (GWO) should orbit toward GSDM Stage 2 during the time period of weeks 2-3. Again, uncertainty is huge including timing, considering the stochastic nature of the dynamical system we are dealing with. However, after a week of very intensive daily monitoring, my confidence in this scenario is increasing. In fact, more and more ensemble numerical models are becoming supportive (NCEP GFS, ERSL/PSD, etc.). In another manner of speaking, I have been offering this option in these discussions for at least a week, and now the numerical models may be starting to catch-up to what diagnostic reasoning suggests as a more probable scenario.

For the PNA sector this suggests the possibility of a major circulation change for especially the USA, meaning an anomalous western USA trough and southeast states ridge. At least for the subseasonal time scale, perhaps the beastly “wicked ridge of the west” will be “dead”. A reversal of the temperature and precipitation anomalies observed across much of the country for the last 4-6 weeks may occur. Internationally, severe thunderstorms and heavy rainfall are probable from southern China into the Philippines at least weeks 1-2. Additional typhoon activity is probable for the TNWP weeks 2-3, possibly impacting locations such as China and Japan. Finally, tropical waves moving west from Africa have been recently intensifying. The risk of tropical cyclone development across the North Atlantic Ocean may increase above climatology going into week 2.

In a somewhat longer term, ~weeks 3-6, I am getting concerned we may start to observe 2 regions of Eastern Hemisphere tropical convective forcing once again, the Indian and west central Pacific Oceans. If the former dominates, GSDM Stage 4-1 would be most probable. Should the GWO orbit to Stage 2 during the next few weeks, it may quickly circuit to the Stage4-1 quasi phase space plane. Stay tuned.

Appendix

An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO), and one of its purposes is to extend thinking beyond the MJO.These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to at least briefly update this Blog on Friday (3 August). In general, due to covering shifts and travel, my postings will be irregular through at least August.